Radio broadcast .. (1922-30)

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Objects That Distort Radio Waves Tests Show that Electric Wires or Cables, Steel Structures, Rivers, Trees, Trolleys, Tennis Backstops, Antennas, and Stone and Iron Monuments All Affect the Direction of Radio Waves By L. E. WHITTEMORE U. S. Bureau of Standards NY one who has thrown a stone into a pond of water knows how the waves spread in all directions from the spot which the stone strikes. The front of the wave moves ang always remaining in the same position rpendicular to the line from the centre of the cle passing out in the direction of travel, is uniform position of the front of the water ve remains until the wave strikes a rock or a stick, or is led down a small channel or bay. Then the direction changes, and the wave takes a position which depends upon the size and nature of the obstacle which it encounters. Similarly, radio waves spread out in all directions from the antenna of an ordinary radio transmitting station. The direction of the front of the wave is constant and is perpendicular to the line of advance of the wave unless some obstacle is encountered, or the wave strikes some new substance which causes it to change its eed or the velocity of transmission. The things which may cause changes in the direction of radio waves are usually objects made of metal. Thus any electric wires or cables or any metallic structure, such as the steel frame of a large building, are likely to cause the direction of the radio waves to change as they travel along the surface of the earth. Even rivers and possibly trees during the spring when the sap is running may affect the direction of the passage of radio waves. A comparison may be made with the waves of light which are transmitted from any object which we are able to see. The light waves ordinarily travel in a straight line, but when they strike some irregular piece of glass they are bent from this straight line. A familiar example is the bending of rays of light by the use of a glass prism or lens. Everyone is familiar with the distortion of light waves caused by a glass of water. A coin or other small object placed in the glass appears, when SP< seen through the water, to be in some position other than its real position. If no objects are in the path of the waves to cause a change in the direction of the front of the wave, one can tell from the position of this wave front the direction from which the wave has come. In the case of the water waves this means the spot where the stone struck the water; in the case of the light waves the luminous object or source of light; and in the case of the radio waves the radio transmitting station. If a small stick were thrown on the water in the region through which the water waves are traveling, this stick might happen to lie in a direction along the front of the wave. It would then move up and down as a whole, first riding on the crest of the wave and then riding in the trough of the wave. If the stick happened to be turned in the other direction, it would bob up and down, one end rising while the other end falls. In this case it would be lying across the line of the wave front, but exactly in line with the direction in which the waves are moving. It is conceivable that such a stick might be used by someone who could not see the water waves, but who could tell by means of his sense of touch by feeling the motion of the stick the direction in which the water waves were traveling. If one cared to determine the direction from which light waves were coming, he could take a hollow tube and look through it as he turns it around. If the same amount of light were visible when one looked through either end of the tube, this tube would then be parallel to the front of the light wave. If as one looked through the tube he saw the light very brightly from one end, but saw no light from the other end, the tube would be across the line of the wave front and in line with the direction of transmission of the light waves. Thus the hollow tube could be used as a direction finder for sources of light Actually we do not have